Position detection apparatus, position detection system, and position detection method

1. A position detection apparatus comprising: a first antenna attached to a predetermined position of a subject and configured to receive a wireless signal transmitted from a medical apparatus inserted into the subject; a second antenna attached to the subject at a different position relative to the predetermined position of the subject to which the first antenna is attached, wherein the different position varies based on a physical form of the subject, and configured to receive the wireless signal; and a processor comprising hardware, the processor being configured to: calculate a first received strength of the wireless signal received by the first antenna and a second received strength of the wireless signal received by the second antenna; calculate an attachment position of the second antenna based on the first received strength and the second received strength; and calculate a position of the medical apparatus based on the attachment position of the second antenna calculated. wherein the processor is configured to select, from a plurality of wireless signals sequentially received, the wireless signal for which the first received strength and the second received strength is calculated, and wherein the processor is configured to, in calculating the attachment position: calculate a ratio of the first received strength and the second received strength; calculate, based on the ratio, an amount of positional change of the second antenna representing a difference between a possible attachment position of the second antenna and a previously stored reference location of the second antenna; and calculate the attachment position of the second antenna based on the amount of positional change of the second antenna calculated.

2. The position detection apparatus according to claim 1 , wherein the processor is configured to: access a memory that stores a reference received strength of each antenna in a previously set reference location of the first and the second antennas, select, from the plurality of wireless signals sequentially received, the wireless signal based on a difference between the reference received strength and the first and the second received strengths.

3. The position detection apparatus according to claim 1 , wherein the processor is configured to select, from the plurality of wireless signals sequentially received, the wireless signal having the first received strength that is greater than a predetermined threshold.

4. The position detection apparatus according to claim 1 , further comprising: a third antenna, wherein the second antenna and the third antenna are attached to the subject at symmetric positions with respect to the first antenna, and wherein the processor is configured to: calculate a third received strength of the wireless signal received by the third antenna; and select, from the plurality of wireless signals sequentially received, the wireless signal of which the second received strength and the third received strength are identical.

5. The position detection apparatus according to claim 1 , wherein the processor is configured to: access a memory that stores a plurality of sets of reference locations of the first antenna and the second antenna; receive an input of information of the physical form of the subject and information about types of the first antenna and the second antenna; and select one of the plurality of sets of reference locations including the previously stored reference location of the second antenna based on the information of the physical form of the subject and the information about types of the first antenna and the second antenna.

6. The position detection apparatus according to claim 1 , wherein the processor is configured to: access a memory that stores a plurality of sets of reference locations of the first antenna and the second antenna; detect types of the first antenna and the second antenna; and select one of the plurality of sets of reference locations including the previously stored reference location of the second antenna based on the types of the first antenna and the second antenna detected.

7. The position detection apparatus according to claim 1 , wherein the processor is configured to: calculate a ratio of the second received strength to the first received strength; calculate an amount of positional change of the second antenna from a predetermined position based on the ratio and correction information, wherein the correction information is a correction function or a look-up table indicating a relation between the ratio of the second received strength to the first received strength and the amount of positional change of the second antenna; and calculate the attachment position of the second antenna based on the positional change of the second antenna calculated.

8. A position detection system comprising: a medical apparatus configured to be inserted into a subject and to transmit a wireless signal; a first antenna attached to a predetermined position of the subject and configured to receive the wireless signal; a second antenna attached to the subject at a different position relative to the predetermined position of the subject to which the first antenna is attached, wherein the different position varies based on a physical form of the subject, and configured to receive the wireless signal; and a processor comprising hardware, the processor being configured to: calculate a first received strength of the wireless signal received by the first antenna and a second received strength of the wireless signal received by the second antenna; calculate an attachment position of the second antenna based on the first received strength and the second received strength; and calculate a position of the medical apparatus based on the attachment position of the second antenna calculated, wherein the processor is configured to select, from a plurality of wireless signals sequentially received, the wireless signal for which the first received strength and the second received strength is calculated, and wherein the processor is configured to, in calculating the attachment position: calculate a ratio of the first received strength and the second received strength; calculate, based on the ratio, an amount of positional change of the second antenna representing a difference between a possible attachment position of the second antenna and a previously stored reference location of the second antenna; and calculate the attachment position of the second antenna based on the amount of positional change of the second antenna calculated.

9. A position detection method comprising: calculating, a first received strength of a wireless signal, transmitted from a medical apparatus inserted into a subject, received by a first antenna attached to a predetermined position of the subject and a second received strength of the wireless signal received by a second antenna attached to the subject at a different position relative to the predetermined position of the subject to which the first antenna is attached, wherein the first position varies based on a physical form of the subject; calculating an attachment position of the second antenna based on the first received strength and the second received strength; and calculating a position of the medical apparatus based on the attachment position of the second antenna calculated, wherein the position detection method further comprises selecting, from a plurality of wireless signals sequentially received, the wireless signal for which the first received strength and the second received strength is calculated, and wherein the calculating the attachment position comprises: calculating a ratio of the first received strength and the second received strength; calculating, based on the ratio, an amount of positional change of the second antenna representing a difference between a possible attachment position of the second antenna and a previously stored reference location of the second antenna; and calculating the attachment position of the second antenna based on the amount of positional change of the second antenna calculated.